There have been a variety of catalytic converters designed for automobile exhaust systems. These converters have included some in which catalyst beads are arranged in an annular bed within the converter to intercept the flow of gas from the engine to the automobile muffler. Some of these converters include designs in which an upstream portion of the exhaust gas first flows into the converter within the annular catalytic bed, then radially through the catalytic bed and finally into a downstream annular flow path outward of the coaxial bed. Such converters are illustrated in U.S. Pat. No. 3,824,790 issued July 23, 1974 in the name of Thomaides, U.S. Pat. No. 3,449,086 issued June 10, 1969 in the name of Innes, U.S. Pat. No. 3,094,393 issued June 16, 1963 in the name of Innes, U.S. Pat. No. 4,208,374 issued June 17, 1980 in the name of Foster, U.S. Pat. No. 3,966,443 issued June 29, 1976 in the name of Okano et al and U.S. Pat. No. 4,148,860 issued Apr. 10, 1979 in the name of Goedicke.
While these patents all appear to provide catalytic converters which reduce harmful substances in the exhaust stream, including unburned hydrocarbons, carbon monoxide and oxides of nitrogen, as well as other components, such designs do have limitations which affect their use. In some of these systems, the catalyst beads abrade during the use of the converter. The abrasion of these catalyst beads significantly reduce the efficiency of the converter over time. The beads abrade because the volume of the catalytic bed expands relative to the volume of the beads when the converter is heated in use. This in turn permits relative movement of the beads as the gas passes through the bed, causing the beads to abrade against one another. Attempts to minimize this abrasion and the consequent attrition of the beads has resulted in efforts to maintain a constant volume in the catalytic bed when the converter is heated. Thus, for example, Innes, U.S. Pat. No. 3,094,394 discloses a relatively elaborate spring mechanism which is intended to maintain a compressive force on the bed as it is heated by the gas flow, thereby maintaining a constant density of the beads in the bed. The Innes reference does not disclose a commercially practical spring mechanism which is capable of withstanding the 1500.degree. F. or more temperature to which these catalytic converters are subjected. In other references inadequate provisions are made for appropriate compensation of the expansion of the volume of the catalytic bed.
In addition, many of these prior art catalytic converters have designs in which noise from the flow of gas is not minimized. Insofar as can be determined, the prior art constructions of the annular outlet in the downstream flow path with cross-sectional areas larger than the cross-sectional area of the inlet end of the upstream portion of the gas flow path. It has been found that in such an arrangement, there is a significant noise level under normal operating conditions generated by gas movement.